1. Field of the Invention
This invention relates to an ohmic contact to a II-VI compound semiconductor device. and its manufacturing method, particularly an ohmic contact to p-type zinc selenium and the method of manufacturing the same.
2. Description of Prior Art
In recent years, II-VI compound semiconductors made of zinc selenium (hereinafter referred to as ZnSe) based materials with wide band gap have been extensively applied in the field of photonics devices, such as photo-detectors, light emitting diodes (LEDs) and semiconductor laser diodes (LDs), etc. Due to a band gap of 2.67 eV at room temperature, the development of blue-green ZnSe-based light emitting device has made considerable progress. Thus, improvement in the low ohmic contacts to these devices becomes increasingly important. Hasse et al. of 3M succeeded in achieving pulsed operation of a II-VI blue-green LD. However, it is difficult to achieve heavy doping in p-type II-VI compound semiconductors. Further, no suitable metal has been found to form good ohmic contacts to II-VI compound semiconductors, and thus Schottky devices must be used.
In view of the large valence band gap, several methods have been proposed to form a good ohmic contact. One method is to find a metal with suitable work function. Namely, n-type semiconductors require metal with low work function and p-type semiconductors require metal with high work function. For ZnSe-based materials with wide band gap, no metal has been found. A second method is to form a heavy doping layer on the surface of the semiconductor, which lowers the Schottky barrier height between the metal and semiconductor. Thus, the carrier can easily tunnel through the shield and improved ohmic behavior is achieved. When n+-type ZnSe is doped by the ZnCl2, it shows improved ohmic behavior.
Fan et al. have obtained a good ohmic contact to p-type ZnSe:N by using a ZnSe/ZnTe multilayer structure for a contact layer to achieve a graded band gap effect; Mensz et al. used a BeTe/ZnSe multilayer structure for a contact layer to achieve a graded band gap effect from p-type ZnSe to p-type BeTe. However, the Be cell must be heated over 1000xc2x0 C., which is difficult. If the epitaxial growth is done by molecular beam epitaxy (MBE), the growth temperature could be about 280xc2x0 C.xcx9c300xc2x0 C. In addition, an optimized contact structure provided by Sony Corp is the ZnSe/ZnTe superlattice. However, it requires much more effort to prepare such a complex superlattice structure compared with a simple metal deposition.
It is an object of the present invention to solve the above mentioned problems by providing an ohmic contact to a p-type II-VI compound semiconductor and obtain a contact resistance as low as the order of 10xe2x88x925xcexa9-cm2. This invention provides a new manufacturing process which forms a metal layer onto p-type or n-type II-VI compound semiconductor to form a good ohmic contact. Further, in order to prevent the metal from oxidizing, a noble metal layer can be formed on the top of the metal layer as a shield layer